Ventilatory response to the PCO2 profile in chicken lungs

Evidence for TREK-like tandem-pore domain channels in intrapulmonary chemoreceptor chemotransduction

Intrapulmonary chemoreceptors (IPC) are carbon dioxide sensing neurons that innervate the lungs of birds, control breathing pattern, and are inhibited by halothane and intracellular acidosis. TASK and TREK are subfamilies of tandem-pore domain potassium leak channels, important in setting resting membrane potential, that are affected by volatile anesthetics and acidosis. We hypothesized that such channels might underlie signal transduction in IPC. We treated mallard ducks with four volatile anesthetics in increasing concentrations to test their effects on IPC discharge through single cell, extracellular recording from vagal fibers. Isoflurane inhalation attenuated IPC discharge only at 8.25% inspired (alpha=0.05). Halothane attenuated IPC discharge significantly (alpha=0.05) at all treatment levels. Chloroform at 3.8%, 5.6%, and 8.25% significantly attenuated IPC discharge (alpha=0.05). Ether at 1.9%, 2.9%, and 3.8% significantly attenuated IPC discharge (alpha=0.05), abolishing IPC discharge at 3.8% inspired. The pharmacological signature of IPC discharge attenuation suggests that IPC express tandem-pore domain leak channels similar to TREK channels, which are inhibited by intracellular acidosis.

Middle cardiac nerve section alters ventilatory response to PaCO2 in the cockerel

To determine whether afferents in the middle cardiac nerves (MCN) contribute to extrapulmonary PaCO2 sensitivity, we did the following: we anesthetized six cockerels with sodium pentobarbital (25-35 mg/kg), and cannulated the cutaneous ulnar vein, and the carotid and brachial arteries. The thorax was opened and each lung unidirectionally ventilated from separate gas delivery systems. A ligature, which temporarily occluded blood flow, was placed around the right pulmonary artery. Both cardiac sympathetic nerves were cut, as well as the left vagus just above the level of the recurrent branch. We exposed the non-perfused right lung to 105 Torr PCO, to silence intrapulmonary chemoreceptors (IPC). We measured blood pressure, heart rate and ventilatory movements while the denervated left lung was used to fix PaCO2 at seven levels ranging from 7-140 Torr. As arterial PCO2 increased, ventilatory amplitude increased from 0.3 mm to 3.6 mm, while frequency decreased from 140 to 24 per min. After cutting the MCN, ventilatory movements were less responsive to PaCO2 changes. Ventilatory amplitude was 3.0 mm at the lowest PaCO2 and increased to 4.0 at the highest PaCO2. We conclude that: 1) when IPC discharge is low, afferents in the MCN inhibit ventilatory movements during hypocapnia, and 2) these afferents may contribute to systemic CO2 sensitivity.

Sensitivity of avian intrapulmonary chemoreceptors to venous CO2 load

To investigate the response of individual intrapulmonary chemoreceptors (IPC) to venous CO2 loads approximating moderate muscular exercise, we recorded vagal discharge from 33 IPC arising from the left lungs of 9 anesthetized, unidirectionally ventilated Pekin ducks. Each IPC was studied during control conditions (PECO2 = 29.0 +/- 0.8 Torr, PVCO2 = 30.2 +/- 0.6 Torr) and during venous CO2 load (PECO2 = 29.5 +/- 0.7 Torr, PVCO2 = 51.5 +/- 1.4 Torr). Venous loading was produced by increasing the percentage of CO2 in the gas ventilating the right lung from 0 to 9-25% CO2. The flow of 1% CO2 through the left lung was adjusted to keep the left lung PECO2 constant. During venous loading, discharge frequencies indicated that the PCO2 at the receptive sites fell, on the average, 1.6 +/- 0.8 Torr.

Receptive fields of intrapulmonary chemoreceptors in the Pekin duck

Reflex experiments indicate a uniform distribution of CO2 chemosensitivity in avian lungs, but neural recording experiments suggest a non-uniform distribution of intrapulmonary chemoreceptor (IPC) endings. To reconcile these observations, blood gases and PECO2 were measured while recording discharge frequencies of 32 IPC innervating the unidirectionally ventilated lungs of 14 Pekin ducks. IPC discharge frequencies, recorded from the left vagus, were determined while ventilating the perfused left lung with caudocranial and craniocaudal flows of 1% CO2 in air, and then while ventilating the unperfused left lung with known levels of CO2 in air. Lung PCO2 profiles were predicted using an eight-compartment computer model of cross-current gas exchange with log-normal ventilation-perfusion inequality and shunt. The PCO2 profiles and IPC discharge frequencies were used to calculate receptor location. At the 99% confidence limit, estimates of IPC location changed significantly in all but 7 IPC when the direction of ventilation was reversed, indicating many IPC have multiple endings. Eighteen of 32 IPC had receptive fields extending at least 50% of the parabronchial length, which may explain the uniform reflex chemosensitivity to intrapulmonary CO2 noted by others.

Effects of PCO2 on respiratory pattern during thermal and exercise hyperventilation in domestic fowl

The relationship between respiratory pattern and arterial PCO2 was investigated during hyperventilation induced by graded exercise and hyperthermia. Treadmill exercise was performed both in isothermic and hyperthermic conditions. Isothermic exercise was induced by spraying the birds with water before exercise at environmental temperatures of 18 +/- 2 degrees C. Hyperthermic exercise was performed in unsprayed birds at temperatures of 18 +/- 2 degrees C and 30 +/- 2 degrees C. During isothermic exercise there was no significant change in arterial PCO2 at moderate work loads and only a small drop in PCO2 at the heaviest work loads; ventilation was increased by coupled increases in tidal volume and respiratory frequency. During exercise in unsprayed birds rectal temperature rose progressively and arterial PCO2 fell progressively with work load. At each work load ventilation was higher and breathing was more rapid and shallow than during isothermic exercise. These effects were more pronounced during exercise at 30 +/- 2 degrees C than at 18 +/- 2 degrees C. When normal PCO2 was maintained during hyperthermic exercise, as a result of the administration of CO2-enriched air, polypnea was suppressed and the tidal volume-respiratory frequency relationship became identical to that observed during graded isothermic exercise. Maintenance of normal PCO2 in resting birds subjected to a gradual increase in environmental temperature also resulted in changes in respiratory pattern identical to those obtained during eucapnic exercise. It is concluded that, provided arterial PCO2 is held constant, the pattern of breathing is the same for hyperventilation induced by exercise or by body temperature increases.